1. Technical Field
The present disclosure relates to communication techniques. In particular, the present disclosure relates to signal processing techniques in a direct-conversion transmitter.
2. Description of the prior art
Main trends for radio frequency (RF) communication systems nowadays are to simplify circuits, to reduce chip size, and to save in overall hardware cost. In comparison with a superheterodyne transceiver, a conventional direct-conversion transceiver has a simpler structure but is not prevalent due to immature manufacturing techniques. In recent years, with the advancements in techniques for manufacturing integrated circuits, a direct-conversion transceiver with good performance becomes practicable and accordingly draws much attention.
FIG. 1 illustrates one part of a conventional direct-conversion transmitter. The direct-conversion transmitter includes a phase lock loop 120, a data generator 140, and a transmitter 100. The phase-locked loop (PLL) 120 includes a phase detector 121, a voltage-controlled oscillator (VCO) 122, and a frequency divider 123. The oscillating signal generated by the voltage-controlled oscillator 122 is divided in frequency by the frequency divider 123, so as to generate a carrier signal. Based on the carrier signal and the data signal provided by the data generator 140, the transmitter 100 generates an output signal. For instance, the transmitter 100 modulates the carrier signal with the data signal in order to generate the output signal.
One problem of this direct-conversion transmitter is that the modulation process in the transmitter 100 usually causes disturbance to the VCO 122. Taking the condition that the divisor of the frequency divider 123 is equal to two (2), the frequency band of the second harmonic of the output signal in the transmitter 100 is very close to the frequency of the oscillating signal in the VCO 122. Similarly, if the divisor of the frequency divider 123 is equal to four (4), the frequency band of the fourth harmonic of the output signal is very close to the frequency of the oscillating signal in the VCO 122. This interference caused by the transmitter 100 introduces phase noise to the oscillating signal in the VCO 122 (i.e. causes the oscillating signal to float) and then inevitably decreases the output signal quality of the transmitter 100.